The ultimate quality of any scanned image is generally limited by the ability of the scanner to resolve minute features in the original object being scanned. A computer image from a scanner contains a large number of computer picture elements, or pixels. The more pixels per unit area in the image, the better its resolution and overall image quality.
High resolution scanning provides faithful reproduction of graphic art material. High resolutions mandate tight production tolerances for critical scanner parameters such as the object-to-image distance (optical focus), the speed and linearity of moving platform and the color accuracy.
Moreover, high resolution scanning is particularly useful when scanning small original objects, such as a frame of 35 millimeter film. Beginning with a resolution of 4000 pixels per inch for a single frame of 35 millimeter film, a relatively high resolution of 500 pixels per inch is maintained when the frame is enlarged to an 8 by 10 inch size of a typical personal computer screen.
Scanners available today are capable of producing images at a single resolution. In other words, today's scanners usually are a compromise between scanning speed and pixel resolution. A scanner today with low pixel resolution can convert an original object to a computer image rapidly, and a scanner with high pixel resolution can convert the same original object but more slowly.
An apparatus for multiple resolution scanning is described in detail hereinafter. An apparatus according to the present invention has a main support, a fine carriage assembly, a main carriage assembly and a camera box assembly.
The fine carriage carries the original object to be scanned. The main carriage carries the camera box which forms the scanned image. The main support also encloses an electronics box which contains circuitry used to control the apparatus and to interface the scanned output image to a host computer.
The scanner is fabricated with heavy-duty, generally expensive materials required by the low-tolerance, high-precision scanning requirements. An outer cover encloses the entire apparatus.
To produce a scanned image, the scanner according to the present invention converts analog light signals to digital pixels along a single fine line across the image in, for instance, the width dimension. This single line across the image is known as a scan line. Many scan lines are built up in, for instance, the length dimension, to form the image.
Scanner resolution capability is measured in pixels per inch, or ppi. The length dimension, in inches, is measured along a single scan line. Typical, scanning resolution for a personal computer application mentioned above is approximately 72 pixels per inch, figured as follows.
Assume that the personal computer screen measures 10 inches in width. The computer hardware produces a scan line of 720 pixels that runs across the 10 inch width. The resulting resolution is 720/10=72 pixels per inch. An arrangement according to the present invention supports resolutions of 667, 1,000, 2,000, 3,000 and 4,000 pixels per inch.
In order to achieve extremely high scanning resolutions of up to 4000 pixels per inch, the object-to-image distance must be precisely controlled to maintain optical focus. That is, a scan line on the original object and the corresponding scan line on the electro-optical imaging device must remain parallel to each other within 9 microns at all times during the scanning operation.
An apparatus according to the present invention, as described in detail hereinafter, contains a unique arrangement of lead screws, pulleys and shafts that control camera box positioning within a 9 micron tolerance for object-to-image distance. Only with such tight tolerances is high resolution scanning possible.
Further, the camera box of the disclosed invention contains a number of individual lenses mounted in a turret assembly. Each lens provides a single scanning resolution.
When a user selects a particular resolution for scanning, the turret assembly moves to place the correct lens between the original object and the electro-optical imaging device. Precision bearings and overall construction of the multi-lens turret assembly also achieves the 9 micron tolerance requirement for object-to-image distance.
The selected lens focuses the light energy from the original object onto a Charge Coupled Device, or CCD. The CCD is a device for converting optical signals into computer pixels at high ppi resolutions. It is a linear array of photodetectors accessed like a shift register with voltage output proportional to light level.
In this case, the CCD has 8,000 triads of photodetectors along its 72 centimeter length, giving the CCD an intrinsic resolution of 2,822 ppi in full red, green, blue (RGB) color. The lenses convert the intrinsic 2,822 ppi CCD resolution to the multiple scanner resolutions of 667, 1,000, 2,000, 3,000 and 4,000 ppi.
In the current invention, combined translational motions from multiple transport platforms produce multiple speed and resolution capabilities. The main carriage uses a stepping motor and timing belt to transport the original object over an 18 inch maximum length to produce low resolution, high speed scans.
The fine carriage uses a precision ball screw mechanism to transport the same original object to produce high resolution, low speed scans. A unique, complementary indexed motion of main and fine carriages supports high resolution scanning over the entire original object.
The fine carriage is not in motion during low resolution scanning. Instead, the main carriage moves the camera box along the entire length of the original object to rapidly form a low resolution scanned image. The original object can be up to 18 inches in length. The main carriage, therefore, translates up to 18 inches in the direction of scan to produce the low resolution output.
The high resolution translation of the fine carriage is limited to approximately 5 inches. This limitation is due to a generally expensive, high precision, zero backlash ball screw which itself is approximately 5 inches in length.
Even though the translational motion of the fine carriage is limited to approximately 5 inches, an entire 18 inch object can be scanned at high resolution. This is accomplished by a complementary, indexed motion of the two carriages, as follows.
First, the fine carriage scans the initial 5 inches of the original 18 inch object by moving it over the camera box on the stationary main carriage.
Second, the main carriage indexes 5 inches along the original object by moving the camera box in the direction of scan.
Third, while the main carriage is indexing, the fine carriage returns to its starting position by moving opposite to the direction of scan. This positions the camera box at the precise location where the scanning left off, but with the fine carriage at its starting point instead of its stopping point.
Fourth, the fine carriage scans the next 5 inches of the original 18 inch object. This process repeats until the entire 18 inch object is scanned at high resolution.
Thus, the present invention describes a unique apparatus which scans long objects at high resolutions using a relatively short precision ball screw drive mechanism.
One modification for a high precision scanner could use a single carriage instead of the multiple carriages described herein. The short 5 inch lead screw in the dual carriage design is replaced by a long 18 inch lead screw in the single carriage design.
Such a long lead screw is required to cover the entire 18 inch scanning area. The single carriage design is unsatisfactory for two reasons.
First, a precision, zero-backlash ball screw is an expensive item with cost proportional to length. An 18 inch precision ball screw would be prohibitively expensive.
Second, the 9 micron depth-of-focus requirement mandates closely spaced re-calibrations of the object-to-image distance during the scanning operation along the entire length of scan. This depth-of-focus (i.e., vertical) calibration is easier to implement using a short 5 inch lead screw, compared with a long 18 inch lead screw.
In the preferred embodiment, both main and fine carriages are driven by stepping motors. One step of the main carriage stepping motor corresponds to 1/2000 inch translation. One step of the fine carriage stepping motor corresponds to 1/12,000 inch translation.
The main carriage translates over the full 18 inch maximum size of the original. The fine carriage translates over the distance of approximately 5 inches allowed by a precision, no-backlash ball screw.
Precision stops along the main carriage provide index points at which the 5 inch fine carriage translation can be reset and a high resolution scan continued over the full 18 inch maximum dimension. The stops along the main carriage are accurate to 2.77 percent of one step of the main carriage, or better than 1/40,000 inch.
The scanner apparatus of the present invention can handle a variety of original objects including 12.times.18 inch, 8.times.10 inch, 4.times.5 inch, 60 mm.times.60 mm and 35 mm transparent film. The maximum width which can be scanned depends on the scan resolution selected.
At low resolution of 667 ppi, the maximum width scanned corresponds to the full width of the carriage, or 12 inches. At higher resolutions, the maximum width decreases as resolution increases to a limit of about 2 inches maximum width at 4000 ppi resolution.
The present invention embodies the benefits of both high speed and high resolution in a single scanning apparatus. Therefore, an original object can be scanned at high speed and low resolution to produce a preview, or working, image.
Also, according to the present invention, an original object can be scanned at low speed and high resolution to produce a final high quality image. This multiplicity of speeds and resolutions is available without tear-down and set-up of the original object that is being scanned.